1. Field of the Invention
The present invention relates to a light-source device and an exposure apparatus. More particularly, the invention is suitably used when manufacturing semiconductor devices, such as DRAMs (dynamic random access memories), LSIs (large-scale integrated circuits) and the like.
2. Description of the Related Art
As semiconductor chips become finer, various light sources, such as high-pressure mercury lamps, KrF excimer lasers and the like, are being used as light sources for reduction-projection-type semiconductor exposure apparatuses. Particularly, in order to improve the throughput in apparatuses having a large angle of view, high-pressure mercury lamps having a higher output have become used as light sources for semiconductor exposure apparatuses using high-pressure mercury lamps. Since most of the electrical power supplied to a light source is converted into thermal energy, the temperature of a high-pressure, high-power mercury lamp becomes very high. Accordingly, in exposure apparatuses, forced air cooling is performed in order to cool a portion of the apparatus that includes a high-pressure mercury lamp, a reflecting-mirror lamp device and a power supply.
Furthermore, in a portion surrounding the lamp device and in an optical path of the exposure light, a very small amount of inorganic gases in the atmosphere reacts with the exposure light to produce a substance that adheres to optical components in the optical path, thereby reducing the transmittance of the optical components and degrading the accuracy during exposure. Accordingly, clean air, which is not chemically contaminated, is required for use as cooling air to cool the lamp device (i.e., the light-source device).
In general, the results of analyzing substances adhering to the optical components indicate that photoreactive substances in the atmosphere of the apparatus comprise NH, SO, silicon compounds (e.g., siloxane), and the like.
These impurity gases slightly remain on the surface of metallic materials during surface treatment, and are contained in the air entering from surrounding portions. It is estimated that, in accordance with a decrease in the wavelength of the exposure light and an increase in the intensity of the exposure light, degradation in the accuracy of the exposure due to the above-described photochemical reaction will be more pronounced.
Accordingly, even if it is desired to increase the brightness (power) of an exposure light source in order to improve throughput, the throughput will become degraded with time, because the above-described impurity gases will gradually reduce the illuminance of the exposure light source.
In order to solve the above-described problem, the lamp device is cooled by blowing compressed clean air, which is used in clean room facilities where the exposure apparatus is installed, onto portions surrounding the lamp and the lamp device, or, in recent exposure apparatuses, by mounting a chemical impurity removing unit, such as a chemical filter or the like, onto the lamp device and using clean air obtained by removing impurity gases by the chemical impurity removing unit.
FIG. 7 is a schematic diagram illustrating the configuration of a semiconductor exposure apparatus disclosed in Japanese Patent Laid-Open Application (Kokai) No. 7-74077 (1995).
In FIG. 7, an exposure apparatus 30 includes a chamber 10, a lamp housing 3, an illuminating optical system 4, a projection optical system 5, a wafer stage 9, a chemical filter unit 31 and the like, and performs exposure transfer of a pattern formed on a reticle 6 onto a wafer 7. A wafer chuck 8 mounts the wafer 7.
In the exposure apparatus 30, air within a clean room where the exposure apparatus 30 is installed is introduced into the chemical filter unit 31, and is discharged to the clean room after being used for cooling the lamp housing 3.
The lamp housing 3 includes a high-pressure mercury lamp (hereinafter abbreviated as a "lamp") 2, an elliptical mirror 11 for reflecting and condensing light emitted from the lamp 2, and a relay optical system 12 for the illuminating optical system 4. The chemical filter unit 31 includes a chemical filter 33, a particle filter 34 and a blower 32, and can adsorb and catch specific chemical substances in passing air. Particularly, in semiconductor exposure apparatuses, it is necessary to filter ammonia gas and hydrogen disulfide gas. Accordingly, clean-room air is introduced into the chemical filter unit 31 by the blower 32, and the clean air passing through the chemical filter 33 and the particle filter 34 is supplied into the lamp housing 3.
The supplied clean air absorbs heat generated from the lamp 2, and is discharged from a heat exhaust port of the clean-room facilities as heat exhaust.
As described above, in conventional semiconductor exposure apparatuses, when cooling the lamp housing 3, the lamp device is cooled by either using high-purity compressed air from the clean-room facilities or a blower unit.
According to such cooling, the temperature of the lamp 2 is maintained within an appropriate temperature range, and heat transfer from the lamp 2 to the main body of the exposure apparatus 30 is prevented. The cooling heat capacity is calculated and the flow rate of cooling air to be supplied is determined so as to satisfy the above-described conditions.
When cooling a lamp by supplying chemically clean air, particularly in a high-output lamp, it is necessary to provide a sufficient amount of air so as not to adversely influence the main body of the exposure apparatus. As a result, the size of the chemical filter provided in the blowing system is increased, the life of the chemical filter is decreased, and it is necessary to frequently exchange the filter. This is not desirable from the viewpoint of the cost.